In a device of the above-indicated type, known from British Pat. No. 1,015,880, a rapidly rotating mixing and kneading tool consists of a shaft which extends lengthwise through the ends of a container and mounts a plurality of radial beaters or knives arranged alongside of and spaced from one another. In the lower part of the container a plurality of stationary knives are arranged which all extend into the spaces between the rotating beaters or knives. In approximately the same plane as the stationary knives, inwardly directed flat projections are provided on the end walls of the container. In the circumferential wall of the container there are two approximately diametrically, opposite pocket-shaped recesses the essentially horizontal surfaces of which offer resistance to the dough carried along by the beaters, thereby effecting a stretching of the dough.
While this action may be somewhat useful for the kneading of the dough, it does not have an effect of the desired extent in the known device as due to the position of the recesses provided therein, the dough deposits firmly in them which not only reduces the desired kneading action but also causes difficulties with the emptying of the container. Another substantial disadvantage, particularly with kneading of firm or tough doughs, is that high shearing forces are produced by the interengagement of the fixed and the rotating knives or beaters. These forces tend to cause an unacceptable overheating of the dough due to friction and to a similarly undesirable repeated tearing of the gluten structure of the dough. Furthermore, there is the danger that parts of the dough will adhere firmly to the drive shaft or on portions of the circumferential wall. Such adhering dough will be carried by the ends of the knives or beaters so that they are not subjected to the actual kneading treatment.
In another type of high-speed kneader, such as disclosed for instance in U.S. Pat. Nos. 1,672,264 and 1,726,033, a mixing and kneading tool rotates horizontally in a U-shaped trough. This tool consists of a continuous shaft having a plurality of parallel kneading bars held by radial arms. In such devices the kneading effect is produced in a manner such that the dough which winds itself about the kneading tool is thrown against the wall of the trough by the centrifugal forces which are generated by the relatively high operating speeds.
As a result of the friction produced thereby, the dough becomes rapidly overheated. In order to avoid this, rotatable deflector means are provided along the wall of the trough to keep the dough away from the wall. This, however, does not eliminate the detrimental effects but only transfers them to a different place as now high shearing forces are produced in the dough when in the vicinity of the deflectors.
British Pat. No. 1262-AD/1880 discloses a dough mixing and kneading device having a U-shaped trough and a mixing and kneading tool without continuous shaft. The mixing and kneading tool consists of three pairs of radial arms of different length. Between each two of these arms there is arranged a grid-shaped construction of longitudinally and transversely extending knives or blades. Such a kneading tool can be operated in practice only at relatively low speeds of rotation. When employed in a high-speed kneader, high frictional forces would be generated entailing detrimental effects on the dough. The centrifugal forces increase as the square of the speed and moreover, the removal of the entire dough from such a kneading tool upon emptying of the device presents considerable difficulties.